Meechelle
I would have thought how you paid for a holiday would not matter. The holiday would still have a cash value, that would be used when claiming. I had a quick look at the Tesco Annual Travel policy and I could not see any reference to clubcard vouchers and how these would be treated when looking at cost of holiday.
I would have thought that if you paid say £700 cash and used say 10,000 worth of vouchers for which £300 value given against holiday, total holiday cost £1000, Insurers would look at £1000 as the cost of holiday and not only £775.
My argument would be that if I used say 10,000 vouchers to buy a TV and the TV was stolen. I would claim against my Household Insurance for the replacement value of the TV. The Insurers could not turn around and say that they will only pay a quarter of the value of the vouchers used. I don't see why Travel Insurance would be treated differently. The idea of Insurance when valid claims are settled it to put you in the same position as before, less the excess amount. If Insurers only gave you £75 and not £300 against the value of the vouchers, this would seem unfair and could be overturned by the FOS if complaint made.
Which company advised you about only paying a quarter of the value of the vouchers? If this was Tesco ask them to clarify which section of their policy advises this.
Huckster